1. Field of the Invention
The present invention relates to a high quality silicon single crystal ingot which is free of point defect, and more particularly, a high quality silicon single crystal ingot and wafer of controlled growth defects having various oxygen concentrations desirable to customers, which are enabled by controlling oxygen concentrations to desirable values when the silicon single crystal ingot is grown by the Czochralski method, and a method and apparatus for growing the same.
2. Description of the Prior Art
As well known in the art, in order to grow a high quality silicon (Si) single crystal ingot that can enhance the yield of semiconductor devices, temperature control has mainly been conducted on high temperature distribution of the single crystal ingot after crystallization. This is done in order to control contraction-induced stress and so on, resulting from cooling subsequent to crystallization or behavior of point defects built up in solidification.
Also, in order to meet various oxygen specifications (suitable for semiconductor devices) demanded by customers, additional capital has been invested. For example, process parameters such as pressure and argon (Ar) flow rate are adjusted, hot zones are changed, and horizontal strong magnetic field is introduced.
According to a typical process of the Czochralski growth of Si single crystal, polycrystalline Si is loaded into a quartz crucible where it is melted into Si melt under the heat radiated from a heater, and then a Si single crystal ingot is grown from the surface of Si melt.
When growing the Si single crystal ingot, the crucible is elevated through the rotation of a shaft that supports the crucible, maintaining the solid-liquid interface at a constant level, and the Si single crystal ingot is wound up while being rotated coaxially with the crucible, but with an opposite rotating direction. Upon being grown as above, the Si single crystal ingot is produced into Si single crystal wafers (via wafer-processing such as slicing, lapping, polishing and cleaning), which are in turn used as semiconductor device substrates.
Conventional techniques for growing a Si single crystal ingot as above have used a heat shield in order to regulate the temperature gradient of a growing Si single crystal and oxygen evaporation from Si melt. Examples of such conventional techniques may include Korean Patent No. 374703, Korean Patent Application No. 2000-0071000 and U.S. Pat. No. 6,527,859 and so on. As, according to a report by Machita et al., “The effects of argon gas flow rate and furnace pressure on oxygen concentration in Czochralski-grown Si crystals” (Journal of Crystal Growth, 186 (1998) 362-368), and Korean Patent Application No. 2001-7011548, installation of a hot zone such as a gas flow controller as well as adjustment of pressure, Ar flow rate and rotation speed of a crucible are proposed as means for controlling oxygen concentration. Furthermore, Japanese Laid-Open Patent Application Nos. 2000-247788 and H10-130100 disclose restraining oxygen dissolution and melt convection by using an apparatus for adjusting magnetic field strength and a unit for generating a multi-CUSP magnetic field.
However, adjustment of several process parameters of the prior art cannot efficiently control the temperature gradient or oxygen concentration of a Si single crystal ingot. So, it has been impossible to produce a high quality Si single crystal ingot and wafer with low point defect concentration that have oxygen concentrations desirable to customers.
Conditions for preferable wafer substrates suitable for device process are as follows: In an active device region formed from a wafer surface to several microlayers in depth, it is preferable to eliminate all agglomerated defects, such as vacancy and self-interstitial, except for point defects. For example, the Crystal Originated Pit (COP), as a type of point defect with agglomerated vacancies worsens the Gate Oxide Integrity (GOI), thereby dropping device yield. Furthermore, GOI may worsen if micro precipitates depending on oxygen and vacancy concentration have occurred in the active device region. On the other hand, Bulk Micro Defect (BMD) containing micro precipitates is needed in a bulk region deeper than the active device region. The MBD occurring during heat treatment of a semiconductor device is harmful for the active device area, but improves device yield by gettering of metal impurities existing in the wafer surface and the active device region. Therefore, a preferable wafer substrate needs suitable vacancy and oxygen concentration.
Meanwhile, as described in Korean Patent Application Nos. 1999-7009261, 1999-7009307 and 1999-7009309, the prior art expresses vertical temperature gradient of crystal in the form of G0=c+ax2. So, vacancy concentration increases gradually to the center from the outer circumference of a single crystal ingot but interstitial concentration decreases. If out-diffusion does not take place by a sufficient degree around the outer circumference of the single crystal ingot, interstitial crystal defect such as LDP occurs. This causes crystal growth to be carried out with high vacancy concentration in the center. Therefore, vacancy crystal defect (e.g., void, Oxidation induced Stacking Fault (OiSF)) tends to occur in the center of a wafer owing to vacancy concentration much higher than balance concentration. On the other hand, dropping the cooling rate of crystal for the purpose of interstitial out-diffusion further requires installation of additional hot zones. This decreases the growth rate of the single crystal ingot, thereby lowering productivity.
As approaches for controlling the temperature distribution of a Si single crystal ingot in order to produce a high quality Si single crystal ingot, following conventional technologies have been proposed. Japanese Patent Application No. H02-119891 proposes to control temperature distribution in the center and circumference of a Si single crystal ingot by adopting hot zones during cooling of the ingot in order to reduce crystal defects in the ingot owing to the strain of solidification. This document particularly discloses using a cooling sleeve to increase solidification rate in the growth direction of the single crystal ingot and thus decreasing lattice defect. Furthermore, Japanese Patent Application No. H07-158458 proposes to control the temperature distribution and pulling rate of a single crystal ingot. Japanese Patent Application No. H07-066074 proposes to improve a hot zone and control cooling rate in order to restrain crystal defect formation by using point defect diffusion. Korean Patent Application No. 2002-0021524 claims that the productivity of a high quality single crystal ingot was enabled by improving a heat shield and a water cooling pipe. Japanese Patent Application H05-061924 proposes to impart periodic variation to the growth rate of a Si single crystal ingot in order to prevent a crystal defect in the single crystal ingot by using the hysteresis of a region where a crystal defect such as OSF and oxygen precipitation defect takes place.
However, the afore-mentioned conventional techniques are based upon solidification or solid state reaction and thus have following problems: First, there are many obstacles against high quality Si single crystal. For example, Korean Patent Application 1999-7009309 (U.S. Patent Application No. 60/041,845) is aimed to reduce point defect concentration by sufficiently diffusing supersaturated point defect in a hot region before it grows into crystal defect. However, temperature has to be maintained up to 16 hours or more for this purpose, and thus this technique can be realized only theoretically but not in actual use.
Second, most conventional techniques fail to make practical achievement. When a 200 mm Si single crystal ingot was grown by periodically varying the pulling speed of a crystal ingot as proposed by Japanese Patent Application H05-61924 and Eidenzon et al. (Defect-free Silicon Crystals Grown by the Czochralski Technique, Inorganic Materials, Vol. 33, No. 3, 1997, pp. 272-279), the high quality target was not achieved, and the process became unstable to the contrary.
Third, those techniques based upon solidification cannot achieve high productivity. Even though a heat shield and a water-cooling pipe were designed in optimal conditions according to the Korean Patent Application No. 2001-7006403, this arrangement merely showed low productivity since pulling speed for high quality single crystal was actually about 0.4 mm/min.
Other approaches for controlling oxygen concentration in a Si single crystal ingot include Japanese Patent Application H10-013010, Korean Patent Registration No. 10-0239864 and Korean Patent Registration No. 2001-7011548. However, these techniques have drawbacks in that either they require additional investment or the high quality single crystal is not actually produced.
Moreover, according to the afore-mentioned techniques, the sought-after high quality single crystal can be produced only with low yield.